I'm sure it will differ from cover to cover, but how many covers can I expect to get out of one cartridge? I have an HPD145 using HP14.

Hard to say. What type of paper do you print on? What quality do you print at (1440, 2880, etc.)? I don't have your printer so I can't give you an exact answer. For my last printer, which was an Epson Stylus Photo 890, I was averaging about 20 covers per ink cartridge. Of course that printer was a serious ink guzzler and HP's typically have a much better ink economy than Epsons. My new printer hasn't been in circulation long enough to get a true approximation yet, but my old old printer was a cheapo Epson Stylus Color 1999 model that I got at least 30 covers per cartridge out of at 1440 dpi on heavyweight Epson photo paper. So if I had to guess, I would say you should be able to get between 20-30 covers at photo quality. Of course that is merely an educated guess since, as I stated, I don't have your particular printer.

On my HP 5550, I get 28 printed before the warning comes up that I'm low on ink. Then I get between 8 and 11 more before it starts to crap out. I print all my covers on HP Glosssy Photo Paper

On a HP 7260 I can get 58 covers and 10 inserts on glossy paper before the color starts fade.